Triggered spark gap type of surge arrestor



March 14, 1967 THOMAS LEE ETAL 3,309,575

TRIGGERED SPARK GAP TYPE OF SURGE ARRESTOR Filed Oct. 19, 1965 5Sheets-Sheet 1 V V LTAGfU/V /BU$ /0 SUM VOLTAGEHON TRIGGER m2 IN TKQD.50 Z 0.L Y T M M i VAG W NM r /OE. A U

March 14, 1967 THOMAS LEE ETAI- 3,309,575

TRIGGERED SPARK GAP TYPE OF SURGE ARRESTOR Filed Oct. 19, 1965 sSheets-Sheet 2 INVENTOR-S.

"v32 Mk THOMAS H. LEE,

TSENG W. L/Ao l 5) ATTORNEY M h 14, 1967 THOMAS H. LEE ETAL 3,309,575

TRIGGERED SPARK GAP TYPE OF SURGE ARRESTOR FiledOct. 19, 1965 5Sheets-Sheet 5 INVENTORS. .THoMAs H. LEE, TSENG W. L/Ao,

BY diam A 770)?NEY United States Patent York Filed Oct. 19, 1965, Ser.No. 505,122

9 Claims. (Cl. 31761.5)

This application is a continuation-in-part of application S.N. 417,704,now abandoned, filed Demmber 11, 1964, and assigned to the assignee ofthe present application.

This invention relates to a triggered spark gap type of surge arrestorfor protecting a D.-C. power system against the effects of voltagesurges and, more particularly, relates to improved triggering means forcausing sparkover of the surge arrestor in response to both lowfrequency and high frequency voltage surges. The invention also relatesto an improved triggering circuit for causing spark-over of the arrestorin response to low frequency voltage surges.

A triggered spark gap type of surge arrestor typically comprises a pairof spaced-apart main electrodes defining a spark gap therebetween andtriggering means for initiating sparkover of the gap when the triggeringmeans is energized with a predetermined minimum voltage. The triggeringmeans typically comprises a trigger electrode located adjacent one ofthe main electrodes but insulated therefrom to provide a trigger gapbetween the trigger electrode and said one main electrode. When asufficient voltage appears across the trigger gap, a spark is developedthereacross, and this releases charged particles which are projectedinto the main gap to initiate a sparkover of the main gap.

In many circuit applications, the triggering means must be capable ofinitiating a sparkover in response to both low frequency voltage surgesand high frequency voltage surges. For example, in certain circuitapplications, the gap, in order to provide the desired protection mustsparkover in response to both lightning surges, which are high frequencysurges, and switching surges, which are of a relatively low frequencycompared to lightning surges.

A very effective triggering means for responding to relatively lowfrequency surges, such as switching surges, comprises a transformerhaving a primary winding connected for energization by the switchingsurge and a secondary winding connected in circuit with the triggerelectrode, When the primary winding is energized by the switching surge,the transformer applies through its secondary winding to the triggerelectrode a voltage pulse that is more effective than the switchingsurge itself would be in producing gap breakdown.

While effective for initiating gap breakdown in response to relativelylow frequency surges, such as switching surges, such a triggering meansis vulnerable to being damaged by high frequency surges, such aslightning surges. Unless the transformer insulation is of exceptionalthickness, this insulation will be damaged by the high frequency surge.7

Accordingly, an object of the present invention is to provide triggeringmeans capable of initiating gap breakdown in response to both low andhigh frequency voltage surges, but which is not vulnerable to damagefrom the high frequency surges.

Another object is to provide triggering means that operates atexceptionally high speed to initiate sparkover of the main gap inresponse to voltage surges of relatively low amplitude on the protectedcircuit.

In carrying out our invention in one form, we provide a surge arrestorthat comprises a pair of spaced-apart main electrodes that are connectedacross the opposite polarity conductors of the D.-C. circuit that is tobe protected.

Each of the main electrodes comprises an arc-initiation portion and anarc-running portion adjacent the arc-initiation. portion. High-frequency-responsive triggering means is provided for causing an arc tobe established between the arc-initiation portions of said mainelectrodes when the high-frequency-responsive triggering means isenergized by a voltage surge on the D.-C. circuit of a predeterminedminimum magnitude having a high rate of change. Thishigh-frequency-responsive triggering means comprises a first triggerelectrode located adjacent the arc-initiation portion of one of the mainelectrodes and insulated therefrom. Low-frequency-responsive triggeringmeans is also provided for causing an arc to be established between thearc-initiation portions of the main electrodes when thelowfrequency-responsive triggering means is energized by a voltage surgeon the DC. circuit of a predetermined min imum magnitude having arelatively low rate of change. This low-frequency-responsive triggeringmeans comprises a transformer and a second trigger electrode in circuitwith the transformer and electrically insulated from the first triggerelectrode. This second trigger electrode is located adjacent thearc-initiation portion of said one main electrode and is insulatedtherefrom. The establishment of an are between the main electrodescompletes a bypass circuit between the opposite polarity conductors ofthe D.-C. system around the transformer and protects the transformerfrom the high rate-of-change voltage surge. Means is provided fordeveloping when an arc is established between the main electrodes anincreasing arc voltage for driving the arcing current toward zero. Thislatter means comprises magnetic means for propelling the are along thearc-running portions of the main electrodes.

For a better understanding of the invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawings; wherein:

FIG. 1 is a schematic view of a surge arrestor embodying one form of ourinvention connected to protect a D.-C. power circuit.

FIG. 2 is a cross-sectional view through an arrestor of the typeschematically depicted in FIG. 1. FIG. 2 is taken along the line 22 ofFIG. 3.

FIG. 3 is a cross-sectional view along the line 3-3 of FIG. 2.

FIG. 4 is a sectional view taken along the line 44 of FIG. 3.

FIG. 4a is a graphical representation of certain voltage relationships.

FIG. 5 is a schematic view showing a modified form or our invention.

FIG. 6 is a sectional view along the line 66 of FIG. 5.

FIG. 7 is a schematic view of a modified form of the invention.

Referring now to FIG, 1, there is shown a D.-C. circuit comprising apositive bus 10, a negative bus 12, and semi-conductor rectifierequipment 14 connected to the buses for supplying D.-C. power thereto.As stated hereinabove, voltage surges, either of a high frequency asmight be produced by lightning or a relatively low frequency as might beproduced by switching, may appear on buses 10, 12, and these surgescould damage the semiconductor equipment 14 unless suitable protectionis provided.

For protecting the equipment 14 from such voltage surges, a surgearrestor, schematically shown at 16 is provided. This surge arrestor hasone terminal 17 connected to the positive bus 10 and its oppositeterminal 18 connected to the negative bus 12, preferably through aresistor 20. The resistor 20 is a non-linear resistor, preferably madeof a material having a negative resistance-current characteristic, suchas the material sold by General Electric Company under the trademarkThyrite.

The illustrated arrestor 16 is, in many respects, identical to thearrestor shown and claimed in our application S.N. 298,942, filed July31, 1963, and will therefore be described in the present applicationonly to the extent believed necessary to convey an understanding of thepresent invention. This arrestor 16 comprises a sealed envelope 21containing an arc-extinguishing gas, preferably one consistingessentially of hydrogen. Disposed within the envelope is a pair ofspaced-apart main electrodes 22 and 24 defining a gap 25 therebetweenacross which arcs are adapted to be established. The electrodes arepreferably of a generally semi-circular configuration with one electrode22 disposed about the other electrode 24. The centers of curvature ofthe two main electrodes are offset with respect to each other so thatthe gap 25 is relatively short in length at one end of the electrodesand gradually increases in length as the other end is approached viacircumferential path extending along the length of the electrodes. Theportion 25a of the gap where the electrodes are closest together isreferred to hereinafter as the arc-initiation region, and the remainderof the electrodes in the arc-initiation region 25a are referred to asarc-initiation portions, and the other electrode portions are referredto as arc-running portions.

Connected in series with the electrodes 22 and 24 are two arc-propellingcoils 28 and 30, one between the ter minal 17 and electrode 22 and theother between the terminal 18 and electrode 24. The coils are used tocreate a magnetic field for propelling the are established between themain electrodes 22 and 24, as will soon be explained.

For initiating an are between the main electrodes 22 and 24, a firsttrigger electrode 132 is provided adjacent the arc-initiation region ofthe main electrode 24. This trigger electrode 132 is separated from themain electrode 24 by means of a strip of high dielectric constantinsulating material 134, preferably of barium titanate. When a voltagepulse of a predetermined minimum amplitude is applied between thetrigger electrode 132 and main electrode 24, the electric field near theedge of the insulating material 134 is intensified due to the highdielectric constant of the insulating material and a spark will jumpacross the trigger gap 133 between the trigge f' 'electrode and the mainelectrode 24. The positive ions produced by the spark distort theelectric field between the two main electrodes 22 and 24, reducing thebreakdown voltage between the main electrodes 22 and 24 to a value belowthe applied voltage between the main electrodes.

This results in an arc being established between the two main electrodes22 and 24 in their arc-initiation region. The current that fiows throughthe are also flows through the arc-propelling coils 28 and 30, and thisproduces a 'magnetic field that drives the arc in the direction of thearrow of FIG. 1, as will soon appear more clearly.

For applying a voltage pulse to the trigger electrode 132 when a surgevoltage appears across the buses 10, 12, an iron core pulse transformer100 is provided. This pulse transformer 100 has a primary winding 102and a secondary winding 104. The primary winding is connected across thebuses 10, 12 and in series with a capacitor 106 located electricallybetween the primary winding and one of the buses 10. The secondarywinding 104 has one terminal which is connected to the negative bus 12and an opposite terminal which is connected through a conductor 105 tothe trigger electrode 132. The trigger gap 133 is, in effect, connectedacross the secondary winding 104.

In a preferred form of the invention, the secondary winding 104 isarranged in such a manner that when the primary winding 102 is energizedby a voltage surge of a predetermined polarity on the bus 10, an outputvoltage pulse of an opposite polarity appears on secondary conductor 105and the trigger electrode 132. This opposite polarity relationship isindicated by the plus and minus signs applied to adjacent terminals ofthe two transformer windings 102 and 104.

For several reasons, the presence of transformer 100 renders theabove-described triggering means more effective in producing a sparkoverof the main gap in response to a voltage surge on bus 10. One of thesereasons is that the transformer 100 is a step-up transformer thatdevelops a higher voltage across its secondary winding than the voltagewhich is applied to its primary winding. Hence, the voltage applied tothe trigger gap 133 through the secondary winding 104 is higher than thesurge volt-' age itself that appears on bus 10. This higher voltage is,of course, more capable of producing a trigger gap sparkover and aresulting main gap sparkover.

A second reason is that the transformer 100 assists in producing a maingap sparkover in the opposite polarity relationship between its outputand input signals. In this connection, note that when the negativepolarity pulse is initially applied to trigger electrode 132 in responseto a positive pulse on bus 10, the main electrode 24 is at the potentialof negative bus 12, since there is no current flowing between mainelectrode 24 and the negative bus 12. When a breakdown of the triggergap 133 occurs in response to this negative pulse, the potential of themain electrode 24 quickly falls to substantially the same value as thenegative potential of the trigger electrode (since the impedance ofcircuit elements 30 and 20 lo-v cated between the main electrode 24 andthe negative bus 12 allows the main electrode 24 to become negative withrespect to bus 12 for a brief period). During this brief period, theother main electrode 22 is at a high positive potential, substantiallyequal to the instantaneous potential of bus 10 with the positive surgethereon, since this electrode 22 is connected directly to the bus 10 andno significant current is yet flowing through the coil 28. The resultantvoltage appearing between the electrodes 22 and 24 across the main gap25 is equal to the arithmetic sum of these two instantaneous voltages,and hence a very high voltage immediately appears across the main gap25, and this accelerates sparkover of the main gap following breakdownof the trigger gap. This arithmetic sum is illustrated at X in FIG. 4a,where the voltages on the various components are depicted just prior tosparkover of the trigger gap.

Had the transformer not reversed the polarity of its output signalrelative to its input signal, the voltage appearing across the main gapimmediately following breakdown of the trigger gap would merely equalthe arithmetic difference of the instantaneous voltages for the two mainelectrodes 22 and 24. Since this difference voltage is much less thanthe previously-described arithmetic sum, it would be less capable ofsparking over the main gap than the net voltage resulting when thenegative polarity pulse is used for triggering. While it is true thatoscillations in the voltage of the lower main electrode 24 occur shortlyafter trigger gap breakdown and these oscillations would result in amuch higher voltage subsequently appearing across the main gap, theabove-described opposite polarity triggering arrangement does not needto wait for such oscillations and can effect an extremely high speedsparkover of the main gap.

The purpose of capacitor 106 connected in series with primary winding102 is to prevent steady-state D.C. current from flowing through theprimary winding, thus preventing the core of the transformer 100 frombeing saturated by the flux that would be generated by such current. Bypreventing such saturation, a smaller core may be used for thetransformer 100.

The above-described triggering means, which includes transformer 100, ifused as the sole triggering means, has a disadvantage of beingrelatively vulnerable to damage by high frequency voltage surgesappearing on the bus 10. Such high frequency voltage surges could reachvery high values across the primary winding 102 before developing asignificant pulse across the secondary winding 104 in view of the timelag inherently present in the transformers operation.

Unless the transformer was built with very special and expensiveinsulation, this high frequency surge across the primary winding coulddamage the transformer insulation before a sufficient secondary pulsedeveloped to initiate breakdown of the main gap.

To protect the transformer 100* from being damaged by these highfrequency voltage surges, we provide a second triggering means thatcomprises a second trigger electrode 32. The second trigger electrode 32is located adjacent the first trigger electrode 132 and the mainelectrode 24 but is insulated from both of these latter two electrodesby the insulation 134 best shown in FIG. 4. A pulse applied to thissecond trigger electrode 32 initiates sparkover of the trigger gap 33between the trigger electrode 32 and the main electrode 24 in the samemanner as described hereinabove with respect to pulses applied to thefirst trigger electrode 132. Sparkover of this trigger gap 33 results incharged particles being projected into the main gap 25 to initiate asparkover of the main gap 25 in the same manner as hereinabove describedwith respect to sparkovers initiated from the first trigger electrode132.

For applying surge voltages to the second trigger electrode 32 when theyappear across the buses 10, 12, the second trigger electrode 32 isconnected to the bus through a small capacitor 36. Under normal orsteadystate conditions, the trigger electrode 32 will be essentiallyisolated from the bus 10 by the capacitor 36. But when surge voltageappears on the bus 10, the capacitor presents a low impedance, and mostof the surge voltage will appear across the trigger gap 33 between thetrigger electrode 32 and the main electrode 24.

The magnitude of the voltage appearing across the capacitor 36 variesinversely with respect to f C, where f is the frequency of the surge andC is the capacitance of capacitor 36. For cost reasons, it is desirableto use a capacitor 36 of low capacitance. The capacitance of capacitor36 is preferably made so low that it is only for relatively highfrequency surges that no substantial voltage appears across capacitor36. Under these high frequency surge conditions, since no substantialsurge voltage appears. across capacitor 36, substantially all the surgevoltage appears across the trigger gap 33 and can spark over the triggergap to initiate breakdown of the main gap. A low frequency surge voltageon the bus 10 causes a much higher percentage of the surge voltage toappear across the capacitor 36 and hence a much lower percentage acrossthe trigger gap. The capacitor 36 is preferably made so small that lowfrequency surge voltages do not develop enough voltage across thetrigger gap to cause it to spark over unless these low frequency surgevoltages reach very high magnitudes beyond the voltage level that it isdesired to protect against.

For triggering the main gap in response to low frequency surges, thefirst triggering means including the transformer 100 is relied upon.This triggering means, as explained above, can initiate gap breakdown inresponse to low frequency surge voltages of any desired low magnitude.High frequency voltage surges acting through the second triggerelectrode 32 can produce a sparkover of the trigger gap 38 and the maingap 25 in a sufficiently short time to prevent the high frequencyvoltage surge from building up an excessively high voltage across theprimary winding 102 of the transformer 100. When the main gap 25 sparksover, it establishes a low impedance circuit through the gap 25 thatshunts the transformer 100 to limit and quickly reduce the voltagedeveloped thereacross, thus protecting the transformer from damagethrough overvoltage.

It will be noted that a resistor 42 is connected between the secondtrigger electrode 32 and the main electrode 24. This resistor 42 has avery low resistance in comparison to the leakage resistance of capacitor36.

The purpose of this resistor 42 is to maintain the trigger electrode 32and the main electrode 24 at substantially the same potential undernormal or steady-state conditions, i.e., conditions when no surgevoltage is present between the buses 10 and 12. Under these conditions,there is a high resistance current path present across the buses 10, 12that comprises the series combination of the leakage resistance ofcapacitor 36, the parallel combination of resistor 42 and the leakageresistance of the trigger gap 33, and the resistance of elements 30 and20. The resistance of elements 42, 30 and is very low in comparison tothe leakage resistance of the capacitor 36. Hence, almost all thesteady-state voltage appears across the capacitor 36, and substantiallynone of this voltage appears across the resistor 42 and, hence, acrossthe trigger gap 33 in parallel with the resistor 42. Isolating thetrigger gap from the steady-state voltage is desirable in preventingdegradation of the trigger gap and possible false sparkovers.

Referring to FIG. 2, it will be noted that the main electrodes 22 and 24are mounted beween two insulating plates 45 that act as side walls forthe arcing gap between the electrodes. These plates 45 are substantiallyimperforate in the region of the arcing gap 25 and extend generallyparallel to the longitudinal axis of any are between the electrodes 22and 24. These insulating plates 45 are made of a material that emitsvery little gas when exposed to an arc, for example, aluminum silicate.The plates 45 are clamped against opposite edges of the electrodes 22and 24 by suitable fastening means such as the insulating bolts 47located at spaced apart locations around the outer periphery of plate45. These bolts 47 extend through aligned openings in the insulatingplates 45 and are threaded into an end cap 48 of the envelope 21.Surrounding each bolt 47 between the plates 45 is a spacer 49 ofinsulating material that limits the clamping pressure applied by the:bolts 47. Also surrounding each bolt is a sleeve 50 that supports theinsulating plates 45 relative to-the end cap 48.

The coils 28 and for creating the arc-propelling magnetic field aremounted on the outer sides of the insulating plates 45. Each of thesecoils is preferably of a circular configuration as viewed in FIG. 3, andhalf of the circumference of each coil is disposed approximately inalignment with the semicircular outer electrode 22. The coils areconnected in the circuit in such a manner that when current flowsthrough the arrestor, it flows through each of the coils in the sameangular direction. Thus, a magnetic field 51 surrounding the two coils28 and 30 and having the general configuration depicted in FIG. 2 isdeveloped. At all-points along the length of the outer electrode 22,this magnetic field 51 extends across the arcing gap 25 in a directiongenerally perpendicular to the longitudinal axis of any are between theelectrodes 22 and 24. As is known, a magnetic field applied transverseto an arc will coact with the local magnetic field around the arc todrive the arc in a direction transverse to the longitudinal axis of thearc and transverse to the direction of the applied magnetic field. Thepolarity of the applied magnetic field in selected so that thearc-propelling force is in the direction of arrow in FIGS. 1 and 3.Thus, when an arc is established at the arc-initiating region 25a, it isdriven along the electrodes 22 and 24 in the direction of arrow 35 tothe opposite end of the electrode.

The motion of the arc in the direction of arrow 35 of FIG. 3progressively lengthens the are due to the progressively increasinglength of the arcing gap 25. This progressive lengthening of the arcproduces a progressive increase in the arc voltage, which progressivelyreduces the arcing current. When the arc voltage exceeds the voltageapplied by the system to the main gap, the arcing current will rapidlyapproach zero. If the energy of the voltage surge that initiated the archas then been dissipated in the arrestor, the arc will be extinguishedand no further breakdown of the gap will occur, thus enabling the systemto be restored to normal operation. It will be apparent that the highestarc voltage is developed when the arc reaches the end of the electrodes22, 24 and is bowed outwardly in its central region, as is shown at 60in FIG. 3. When in this position, the arc has its maximum length.

If the voltage surge is a high energy surge, only a portion of the surgeenergy will have been dissipated by the time the arc reaches itsposition 60 of FIG. 3. The remaining surge energy will produce anotherabrupt voltage rise that will cause the main gap to spark over in thearc-initiation region 25a, thus establishing another arc between themain electrodes in the arc-initiation region 25a. The first arc may ormay not have been completely extinguished at the instant that the secondarc is established, but upon establishment of the second arc, the firstarc vanishes. The second arc, like its predecessor, is driven in thedirection of arrow into position 6t thereby increasing the arc voltageand driving the are current rapidly towards zero. Just before or as soonas the current reaches zero, the surge voltage resulting from theremaining surge energy initiates a third are in the arcinitiation region25a. The second arc vanishes, and the third are is handled in the samemanner as its predecessor. This sequence of events is repeated over andover again until the surge energy is finally completely dissipated. Whenthis complete dissipation occurs, the maximum arc voltage developed whenthe arc is at position 6! is insufficient to cause a breakdown at thearc-initiation region 25a, and hence the gap acts thereafter to preventfurther current flow.

If the type of high frequency voltage surge that the arrestor is toprotect the circuit against is a lightning surge and particularly thetype of lightning surge that results from a lightning stroke to thesystem 10, 12 at a point near the arrestor, then it is most desirablethat the arrestor be constructed generally as shown in our applicationS.N. 397,215 filed September 17, 1964, now Patent No. 3,287,588. Such anarrestor is illustrated in FIG. 5.

The current through an arrestor that accompanies a lightning surgecomprises two parts: (1) a lightning discharge current, which is thecurrent of the lightning surge, and (2) a follow current, which is thecurrent of the system that flows through the arrestor following passageof the lightning discharge current. The magnitude of the lightningdischarge current is largely independent of the impedance of thearrestor and therefore may reach very high values. If an are carrying avery high current were forced from the arc-initiating region 25a in thedirection of arrow 35, as described in connection with FIGS. 1-4, anexcessively high are voltage would be developed. In this respect, thedischarge path between the electrodes 22 and 24 at the left hand side ofthe arcinitiating region in FIG. 5 is the same as that of the arrestorof FIGS. 14 and therefore has a relatively high impedance. For lowcurrent arcs such as switching surge arcs, this high impedance isdesirable because it enables the arc voltage to be built up quickly toforce the switching surge current toward zero. The flow of switchingsurge current through this relatively high impedance path does notdevelop excessive voltages across the arrestor because the switchingsurge current is relatively low and is limited by the relatively highimpedance of the arrestor. But lightning discharge currents will usuallybe much higher and will have a magnitude that is essentially independentof the arrestor impedance. Accordingly, if this high lightning dischargecurrent was discharged through the high impedance path at the left handside 25b of the arrest-0r, excessive voltages would be developed acrossthe arrestor that could damage the rectifier equipment 14.

To prevent the development of such excess voltages, we exclude highlightning discharge current arcs from the left hand region 25b of thearrestor and instead propel these arcs from the arc-initiation region25a into a region E: 256 at the right of the arc-initiation region. Forreasons which will soon be explained, the right hand region 250 of thearrestor has a relatively low impedance. Hence, the passage of the highlightning discharge currents through this path does not generateexcessive voltages across the arrestor.

The reason that the right hand portion 250 of the arrestor has arelatively low impedance compared to that of the left hand portion 25bis that the spacing between the insulating side walls 45 in this regionis relatively large compared to the spacing in the region 25b. Thisrelatively large spacing of the side walls 45 permits any are burning inthis region 250 to increase its cross section and to become diffused,which in turn permits it to burn with a much lower arc voltage. Ineffect, this region 25c of relatively large side wall spacing presents alow impedance path for any lightning discharge current are which ispropelled into it.

For propelling a high current lightning arc in the direction of arrow 37(FIG. 5) from the arc-initiation region 25a into the low impedanceregion 25c, we disable the lower coil 30 by shorting it out (in a mannersoon to be described) and permit the magnetic field from the upper coil28 to propel the lightning discharge current arc. The field from thisupper coil 28 has a polarity such as to drive arcs in the direction ofarrow 37, and accordingly the lightning discharge current are will bedriven in the direction of arrow 37. The coil 28 has only a smallpercentage of the number of turns of coil 30 and normally itsarc-propelling ability is completely defeated by the opposing magneticfield of the coil 30. But when the coil 30 is disabled, the magneticfield from the upper coil is capable of forcing an are established atthe arc-initiation region toward the right. Even though the coil 28 hasonly a few turns, it can provide a high enough magnetic field toeffectively propel the lightning current are because the lightningcurrent that traverses the coil during this interval is very high. It ismost desirable that this coil 28 have a minimum number of turns sincethis limits its impedance to a sufficiently low value to preventexcessive voltages from being developed thereacross by the lightningcurrent.

For disabling the other coil 30 during the period when lightningdischarge current is flowing through the arrestor, we provide acoil-shorting gap 70 that is connected in parallel with the coil 30.Since both the magnitude and rate of change of lightning dischargecurrent are very high and since the coil 30 has a relatively largenumber of turns, the voltage developed across the coil 30 by thelightning current quickly rises toward a high value. This sharply risingvoltage is used to spark over the coil shorting gap 70, and thereafterthe lightning current flows through the coil shorting gap 70. The coilshorting gap 70 is designed to present a low impedance to the lightningcurrent, and thus the voltages developed thereacross by the lightningcurrent are limited to a relatively low value.

This coil-shorting gap 7% comprises spaced-apart electrodes 72 and 74defining a gap 75 therebetween and an arc-propelling coil 73 forpropelling an arc across gap 75 in the direction of arrow 77. The gap 70is constructed in the same manner as a similarly designated gap in ouraforesaid application Serial No. 397,215 and will, therefore, not bedescribed in detail in the present application.

When the lightning discharge current has fallen to a predetermined valueindicative of substantially complete dissipation of the energy of thelightning surge, the arc in the coil-shorting gap 70 will beextinguished. The follow current that flows thereafter follows a paththrough the coil 30 rather than the gap 70. This is the case because thecoil 30 presents a very low impedance to the follow current in view ofthe low rate of change of the follow current. Since this impedance tofollow current through the coil 30 is much lower than that through thecoil shorting gap 70, essentially all of the follow cur- 3 rent flowsthrough the coil 30 after passage of the lightning discharge current.

As soon as the coil 30 is traversed by follow current,

it develops its previously-described magnetic field for driv-' ing thearc in the main gap in the direction of arrow 35. The are in the maingap is then carrying follow current. This are is forced by the magneticfield of the lower coil 30 into the left hand region 25b of gap 25 wherethe spacing between the insulating plates is small. This results in thedevelopment of a higher are voltage and higher effective impedance,which drives the current through the arrestor to zero and preventsreestablishment of the are, all in the same manner as describedhereinabove with respect to FIGS. 14. Typically, the arc carrying powerfollow current after the passage of the lightning discharge current canbe extinguished even before it has reached the position 60 of FIG. 3 onits first movement through the arc-running region 25b.

The arrestor of FIG. 5 utilizes substantially the same two triggeringmeans as the arrestor of FIGS. 1-4, and corresponding parts of thesetriggering means have been assigned identical reference numerals.Lightning surges, which are high frequency surges, will initiatearc-over of the arrestor by triggering it through the trigger means 32,3-6. Switching surges, which are relatively low frequency surges, willinitiate arc-over of the arrestor by triggering it through thetriggering means 132, 100.

When the main gap is sparked over by a voltage pulse applied through thetriggering means 32, 36 in response to the high frequency lightningsurge, the arc that is initially formed is driven into the low impedanceregion 250 of the arrestor to the right of the arc-initiation region.After the lightning current is dissipated, the are carrying followcurrent is driven into the relatively high impedance region 25b of thearrestor to build up a high are voltage that extinguishes the arc.

When the main gap is sparked over by a voltage pulse applied through thetriggering means '100, 132 in response to a relatively low frequencysurge such as a switching surge, the are that is initially formed isdriven to the left into the relatively high impedance region 25b toquickly build up are voltage for extinguishing the arc, as describedhereinabove.

FIG. 7 illustrates a slightly modified form of triggering means forcausing spark-over of the surge arrestor in response to a low frequencyvoltage surge. In many respects, the embodiment of FIG. 5 corresponds tothat of FIG. 1, and, hence, corresponding parts of these embodimentshave been assigned corresponding reference characters. The basicstructural diiference between these two embodiments is that in FIG. 5 acapacitor 150 has been connected across the secondary winding 104 of thepulse transformer 100. a

This capacitor 150 serves a number of important functions. One is thatit prevents the iron core of the pulse transformer 100 from becomingsaturated by the cumulative build-up of residual magnetism therein as aresult of repeated unidirectional voltage pulse appearing across buses10, 12 over a prolonged period. Without the capacitor 150, theapplication of these repeated unidirectional voltage pulses to theprimary winding 102 has a tendency to cumulatively build-up residualmagnetism in the iron core. As this cumulative build-up continues andthe saturation point is approached, the transformer 100 loses itsability to produce an output pulse of the desired wave shape andamplitude. The capacitor 150 prevent-s this condition from developinginasmuch as it cooperates with the secondary winding 104 to form anoscillatory circuit. This oscillatory circuit produces an oscillatoryoutput voltage on the trigger electrode 132 in response to aunidirectional voltage pulse being applied to primary winding 102 and,furthermore, produces this oscillatory voltage irrespective of whetherthe trigger gap 133 sparks over. The oscillatory current flowing throughthe secondary winding 104 as a result of the oscillatory voltagedeveloped by the circuit 104, produces a countermagnetizing force whichdrives any residual magnetism in the core back to approximately zero.Thus, the core of the transformer is, in effect, reset by theoscillatory current and is thus prevented from accumulating suflicientresidual magnetism to impair operation of the pulse transformer.

Another function of the capacitor 150 is to extend the period that themain electrode 24 remains near the peak pulse voltage of the triggerelectrode 132 following spark over of the trigger gap 133. Thisextension results from the added energy stored in the capacitor 150,which decreases the rate at which the voltage of the main electrode 24falls following trigger gap spark over. By extending this period, avoltage substantially equal to X in FIG. 4a is maintained for anextended period, thus increasing the chances for a main gap spark-overin response to a trigger gap spark-over.

Another function of capacitor 150 is to increase the current and energysupplied to the trigger gap 133 upon its sparkover, thereby releasingmore charged particles to accelerate spark-over of the main gap 25a.

In typical embodiments of our invention, the capacitor 150 has acapacitance of 0.001 to 0.01 microfarad; the capacitor 106 has acapacitance of 1 microfarad; and the pulse transformer has a ratio ofbetween 1.5 and 3 to 1. The larger the ratio of the transformer, thesmaller the value of capacitance used for capacitor 150.

In its broader aspects, our invention contemplates use of thelow-frequency triggering circuits of FIGS. 1 and 7 either in combinationwith a high-frequency triggering circuit, as shown in FIG. 1, or withoutthe high-frequency triggering circuit.

While we have shown and described particular embodiments of ourinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from ourinvention in its broader aspects and we, therefore, intend in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a D.-C. circuit comprising a pair of conductors of oppositepolarity, a surge arrestor comprising:

(a) a pair of spaced apart main electrodes,

(b) means adapted to electrically connect said main electrodes acrosssaid conductors,

(c) each of said main electrodes comprising an arcinitiation portion andan arc-running portion adjacent said arc-initiation portion,

((1) high-frequency-responsive triggering means comprising a firsttrigger electrode located adjacentthe arc-initiation portion of one ofsaid main electrodes for causing an arc to be established between thearc-initiation portions of said main electrodes when saidhigh-frequency-responsive triggering means is energized by a voltagesurge on said D.-C. circuit 7 of a predetermined minimum magnitudehaving a high rate of change,

(e) low-frequency-responsive triggering means for causing an arc to beestablished between the arcinitiation portions of said main electrodeswhen said low-frequency-responsive triggering means is energized by avoltage surge on said D.-C. circuit of a predetermined minimum magnitudehaving a relatively low rate of change, said low-frequency-responsivetriggering means comprising a transformer and a second trigger electrodein circuit with said transformer and electrically insulated from saidfirst trigger electrode, said second trigger electrode being locatedadjacent the arc-initiation portion of said one main electrode andinsulated therefrom,

(f) the establishment of an arc between said main electrodes completinga by-pass circuit between said D.-C. conductors around said transformerthat protects said transformer from said high rate of change voltagesurges,

(g) and means operable when an arc is established between said mainelectrodes for developing an increasing arc voltage for driving the arccurrent toward zero, comprising means for propelling said arc along thearc-running ortions of the main electrodes.

2. The combination of claim 1 in which said transformer is a step-uptransformer for increasing the magnitude of the voltage pulse applied tosaid trigger electrode as compared to the magnitude of the voltage surgeon said D.-C. circuit.

3. The combination of claim 1 in which:

(a) said transformer comprises a primary winding and a secondarywinding,

(b) means is provided for connecting said primary winding forenergization by voltage surges on said D.-C. circuit,

(c) means is provided for connecting said secondary winding in circuitwith said second trigger electrode,

(d) the polarity of said secondary winding with respect to said primarywinding is such that when the primary winding is energized by voltagesurge on said D.-C. circuit, a voltage pulse of opposite polarity tosaid voltage surge appears across said secondary winding and is appliedto said second trigger electrode, and

(e) means is provided for causing substantially the arithmetic sum ofthe voltages of said voltage pulse and said voltage surge to be appliedbetween said main electrodes immediately following sparkover betweensaid second trigger and said one main electrode.

4. The combination of claim 1 in which:

(a) said transformer comprises a primary winding and a secondarywinding,

(b) means is provided for connecting said primary winding forenergization by voltage surges on said D.-C. circuit,

() means is provided for connecting said secondary winding in circuitwith said second trigger electrode, and

(d) the polarity of said secondary winding with respect to said primarywinding is such that when the primary winding is energized by voltagesurge on said D.-C. circuit, a voltage pulse of opposite polarity tosaid voltage surge appears across said secondary winding and is appliedto said second trigger electrode.

5. The combination of claim 1 in which said highfrequency-responsivetriggering means comprises a capacitor connected between said firsttrigger electrode and the other of said main electrodes, said capacitorbeing so small that low rate-of-change voltage surges that causeoperation of said low-frequency-responsive triggering means normally donot cause operation of said highfrequency-responsive triggering means.

6. In a D.-C. circuit comprising a pair of conductors of oppositepolarity, a surge arrestor comprising:

(a) a pair of spaced-apart main electrodes,

(b) means adapted to electrically connect said main electrodes acrosssaid conductors,

(c) each of said main electrodes comprising an arcinitiation portion andan arc-running portion adjacent said arc-initiation portion,

((1) triggering means comprising a trigger electrode located adjacentone of said main electrodes and insulated therefrom for causing an arcto be established between the arc-initiation portion of said mainelectrodes when said triggering means is energized by a voltage surge onsaid D.-C. circuit of a predetermined minimum magnitude,

(e) said triggering means further comprising:

(i) a transformer having primary and secondary windings,

(ii) means for connecting said primary winding for energization byvoltage surges on said D.-C. circuit,

(iii) means for connecting said secondary winding in circuit with saidtrigger electrode,

(f) the polarity of said second winding with respect to said primarywinding being such that when the primary Winding is energized by avoltage surge on said D.-C. circuit, a voltage pulse of oppositepolarity to said voltage surge appears across said secondary winding andis applied to said trigger electrode, and

(g) means for causing substantially the arithmetic sum of the voltagesof said voltage pulse and said voltage surge to be applied between saidmain electrodes immediately following sparkover between said triggerelectrode and said one main electrode.

7. The apparatus of claim 1 in which:

(a) said main electrodes having auxiliary portions located on theopposite side of said arc-initiation portion from said arc-runningportions,

(b) means is provided defining a region of relatively low impedance toarcing current flowing between said auxiliary portions in comparison tothe impedance to arcing current flowing between said arcrunningportions,

(c) means is provided for driving arcs initiated by saidhigh-frequency-responsive triggering means into said region ofrelatively low impedance, and

(d) means is provided for driving arcs initiated by saidlow-frequency-responsive triggering means along said arc-runningportions without entry of said latter arcs into said region ofrelatively low impedance.

8. The apparatus of claim 6 in combination with a capacitor connectedacross said secondary winding for substantially increasing the currentand energy supplied to the trigger gap upon its sparkover as compared tothat available from said transformer without said capacitor.

9. The apparatus of claim 6 in which:

(a) said transformer comprises an iron core, and

(b) oscillation-producing means is provided for causing the voltageappearing across said secondary winding when said primary winding isenergized by a unidirectional voltage surge to be an oscillatory voltagecapable of preventing a cumulative build-up in residual magnetism insaid iron core as a result of repeated unidirectional voltage surgesbeing applied to said primary winding,

(c) said oscillation-producing means comprising a capacitor connectedacross said secondary winding.

References Cited by the Examiner UNITED STATES PATENTS 656,681 8/1900Thomson 3l7-61.5 X

5 MILTON O. HIRSHFIELD, Primary Examiner.

J. D. TRAMMELL, Assistant Examiner.

1. IN A D.-C. CIRCUIT COMPRISING A PAIR OF CONDUCTORS OF OPPOSITE POLARITY, A SURGE ARRESTOR COMPRISING: (A) A PAIR OF SPACED APART MAIN ELECTRODES, (B) MEANS ADAPTED TO ELECTRICALLY CONNECT SAID MAIN ELECTRODES ACROSS SAID CONDUCTORS, (C) EACH OF SAID MAIN ELECTRODES COMPRISING AN ARCINITIATION PORTION AND AN ARC-RUNNING PORTION ADJACENT SAID ARC-INITIATION PORTION, (D) HIGH-FREQUENCY-RESPONSIVE TRIGGERING MEANS COMPRISING A FIRST TRIGGER ELECTRODE LOCATED ADJACENT THE ARC-INITIATION PORTION OF ONE OF SAID MAIN ELECTRODES FOR CAUSING AN ARC TO BE ESTABLISHED BETWEEN THE ARC-INITIATION PORTION OF SAID MAIN ELECTRODES WHEN SAID HIGH-FREQUENCY-RESPONSIVE TRIGGERING MEANS IN ENERGIZED BY A VOLTAGE SURGE ON SAID D.-C. CIRCUIT OF A PREDETERMINED MINIMUM MAGNITUDE HAVING A HIGH RATE OF CHANGE, (E) LOW-FREQUENCY-RESPONSIVE TRIGGERING MEANS FOR CAUSING AN ARC TO BE ESTABLISHED BETWEEN THE ARCINITIATION PORTIONS OF SAID MAIN ELECTRODES WHEN SAID LOW-FREQUENCY-RESPONSIVE TRIGGERING MEANS IS ENERGIZED BY A VOLTAGE SURVE ON SAID D.-C. CIRCUIT OF A PREDETERMINED MINIMUM MAGNITUDE HAVING A RELATIVELY LOW RATE OF CHANGE, SAID LOW-FREQUENCY-RESPONSIVE TRIGGERING MEANS COMPRISING A TRANSFORMER AND A SECOND TRIGGER ELECTRODE IS CIRCUIT WITH SAID TRANSFORMER AND ELECTRICALLY INSULATED FROM SAID FIRST TRIGGER ELECTRODE, SAID SECOND TRIGGER ELECTRODE BEING LOCATED ADJACENT THE ARC-INITIATION PORTION OF SAID ONE MAIN ELECTRODE AND INSULATED THEREFROM, (F) THE ESTABLISHMENT AND INSULATED THEREFROM, ELECTRODES COMPLETING A BY-PASS CIRCUIT BETWEEN SAID D.-C. CONDUCTORS AROUND SAID TRANSFORMER THE PROTECTS SAID TRANSFORMER FROM SAID HIGH RATE OF CHANGE VOLTAGE SURGES, (G) AND MEANS OPERABLE WHEN AN ARC IS ESTABLISHED BETWEEN SAID MAIN ELECTRODES FOR DEVELOPING AN INCREASING ARC VOLTAGE FOR DRIVING THE ARC CURRENT TOWARD ZERO, COMPRISING MEANS FOR PROPELLING SAID ARC ALONG THE ARC-RUNNING PORTIONS OF THE MAIN ELECTRODES. 